Alzheimer's disease (AD) is a common neurodegenerative disorder causing dementia and a major cause of death. In AD there are three major microscopic features that are recognized as the hallmarks of the disease, namely, neuritic plaques (NP), neurofibrillary tangles (NFT) and amyloid angiopathy (AA). In addition, there is widespread cell loss, particularly of cholinergic neurons in the brain. Loss of cholinergic cells leads to reductions in the levels of the neurotransmitter acetylcholine (ACh), its synthesizing enzyme choline acetyltransferase, as well as its deactivating enzyme actetylcholinersterase (AChE). Reduction of cholinergic neurotransmission leads to some of the symptoms of AD. Because of this reduced cholinergic activity, drugs such as donepezil, metrifonate, rivastigmine, huperzine A, and tetrahydroaminoacridine, which inhibit the activity of cholinesterase and increase the level of ACh, have been used to treat the symptoms (though not the disease progression) of AD.
Determining when to begin therapy for AD is an extremely difficult task, however, because confirmation of the diagnosis of dementias such as AD is difficult at present and can only be truly confirmed by autopsy. Distinguishing between different forms of dementia such as AD, dementia with Lewy bodies, vascular dementia and frontotemporal dementia is challenging. Furthermore, early diagnosis of incipient dementia, such as mild cognitive impairment, becomes imperative for beginning symptomatic therapy, as well as when new disease-modifying AD drugs are developed. Thus, there remains a great need for a means of identifying and diagnosing AD at an early stage.
Neuroimaging is increasingly used to assist in diagnosis, but no satisfactory diagnostic tool has heretofore emerged. Structural imaging such as Computer Assisted Tomography (CAT) and Magnetic Resonance Imaging (MRI) provide information about changes in the brain, such as atrophy, stroke, malignancy and white matter changes, but at a gross anatomical level. Functional imaging using Single Photon Emission Computer Tomography (SPECT) and Positron Emission Tomography (PET) are non-specific. For example, one of the PET ligands used is 18-fluorodeoxyglucose and, for SPECT, 99mTc hexa-methyl-propyl-amino oxime (HMPAO) or 99mTc ethylene dicysteine di-ethyl ester (ECD). Each of these techniques provides information regarding reduced functional integrity of different parts of the brain, but do not provide information as to why there is reduced function in these regions. More recently, radioligands that bind to the AD-implicated β-amyloid protein have been developed (for example, AV-45). These ligands provide information as to whether, and where, β-amyloid is present in the brain. From a neuropathologic perspective, deposition of amyloid and formation of NP is one of the central mechanisms in the evolution of AD. However, amyloid plaques are also found in brains of elderly individuals who do not have dementia, which limits the applicability of this approach.
A technique of greater utility involves binding to cholinesterases, especially butyrylcholinesterase (BuChE). Although the level of AChE is reduced in AD, the level of the closely related enzyme BuChE is increased in AD brains. There is a severe loss of basal forebrain cholinergic neurons in AD (Coyle J. T., Price D. L., and DeLong M. R., 1983, Science 219:1184-90), and there is a marked decrease in the levels of AChE and a similarly marked increase in the levels of BuChE (Perry E. K., Perry R. H., Blessed G., and Tomlinson B. E., 1978, Neuropath. Appl. Neurobiol. 4:273-277; Mesulam M-M. and Geula C., 1994, Ann. Neurol. 36:722-727). BuChE is found in all the neuropathological lesions associated with AD (NP, NFT and AA). Amyloid plaques in individuals without dementia are “benign” and they become “malignant”, causing dementia, when they are transformed into NP. Importantly, BuChE is found in NP in brains of patients with AD rather than plaques found in brains of elderly individuals without AD. Taken together, these observations show that in brains of patients with AD there is a significant alteration of the biochemical properties of BuChE that alters its normal regulatory role in the brain (Guillozet A. L., Smiley J. F., Mash D. C., and Mesulam M-M., 1997, Ann. Neurol. 42:909-918). As such, high affinity labeling of BuChE, particularly with radioligands, has utility in the early diagnosis of AD.
Within the context of detecting BuChE in the brain, only one experimental radioligand is available, namely, 1-[11C]-methyl-4-piperidinyl n-butyrate. However, this compound has disadvantages. First, it is labeled with 11C, emitting positrons that can only be detected by PET scanners, which are not as widely available as SPECT scanners. Second, the radioactive label is attached to the molecule in the portion that is an initial leaving group in the mechanism of the enzyme catalyzed hydrolysis of this radiopharmaceutical. Thus, the radioactive atom intended to label the enzyme is lost in the early stages of the reaction, which would lead to short-lived enzyme-detecting capability of this ligand and only show diffuse distribution of the label rather than specific regional distribution of the target enzyme.